A quasiclassical and approximate quantum mechanical study of the intramolecular isotope effect in proton transfer elementary reactions: the Ne + HD+ → NeH+(NeD+) + D(H) system at low and moderate collision energies (0.02–0.77 eV)

Abstract

The quasiclassical trajectory method (QCT) and the quantum mechanical (QM) reactive infinite order sudden approximation (R-IOSA) method have been applied to study the dependence of the isotope effects in the Ne + H2+ reaction with the relative translational energy (Etras) and vibrational energy of the molecular ion, dedicating particular attention to the intramolecular isotope effect. To do this, we have used two different analytical functions to represent the ground NeH2+(2A′) potential energy surfaces (PES). The intramolecular isotope branching ratio is essentially constant and equal to unity in the Etras range considered (0.02–0.77 eV) and the QCT results are in good agreement with a mass spectrometric experiment. The R-IOSA results also agree with the experimental data but at higher collision energies. However, the theoretical approach, using a single PES, to the reaction generated after the chemiionization of the HD molecule by a metastable Ne atom has been unable to reproduce the experimental intramolecular isotope effect value, σ(NeH+)/σ(NeD+), of 0.67. A detailed analysis of the QCT reaction dynamics at low Etras (0.02–0.08 eV) for both reaction channels, considering scalar and vector properties and carried out using the most accurate PES available, suggests that at the very low Etras values mainly involved when the reaction arises from chemiionization of HD, the Ne + HD+ dynamics is essentially unimolecular and the density of states play a dominant role in the intramolecular branching ration, favoring the formation of the NeD+ product over the NeH+ one.